The manufacture of coextruded blown films and the equipment for making them are well known in the art. Multilayer films have been made in which at least one surface or outer layer are made to facilitate heat-sealing. A core film layer may be used to provide strength, impact resistance, stretchability, other main physical properties of the film, or combinations thereof. Layers between the outer layer and core layer, also referred to as inner layers in this context, may facilitate the mutual adhesion of the layers and/or may impart barrier properties against the transmission of moisture, carbon dioxide, oxygen, other gases and the like.
Polymers used in such processes for packaging applications generally include polyethylene, polypropylene, ethylene vinyl alcohol, and the like. Film properties are often subject to the combined effect of the coextrusion process conditions and polymer compositions selected for the different layers. Film producers have to balance mechanical properties such as stiffness and impact strength to make stronger films for a given thickness, and optical properties such as clarity and haze which impact the attractiveness of the packaging and visual inspection of the goods at the point of sale.
Higher clarity is usually achieved in blown film applications by the inclusion of softer polyolefins, such as very low density polyethylene (LLDPE), usually in combination with other polyethylene polymers, including low density polyethylene (LDPE), linear polyethylene, and high density polyethylene (HDPE), in different layers. Such materials, while offering benefits in clarity, often result in negative effects on the mechanical properties. Various attempts to optimize this balance continue to restrict the range of stiffness. Introduction of higher amounts of metallocene polyethylenes (mPEs) into multilayer films with reduced use of LDPE provides improved mechanical performance. Efforts to provide a good solution for high clarity films include incorporating mPE with a higher density, optionally in a blend with high density polyethylene (HDPE), in the core layer and another mPE with a lower density in the outer layer. Although improved optical properties including high clarity and low haze are achieved, stiffness of the film is usually very high, making the film undesirable for packaging applications requiring soft hand feel.
U.S. Pat. No. 6,368,545 discloses techniques to achieve higher clarity in multilayer blown coextruded films. The core layer may be extruded at a higher temperature than the skin layer or layers, and/or the core layer has a higher density than the skin layer or layers.
U.S. Patent Application No. 2009/0110913 provides a blown film coextrusion process. The process involves extruding core contact layers through a die exit at a specified speed and cooling, orienting, and crystallizing the multi-layer film formed at a specified deformation rate to form a frost line at a specified distance from the die exit by withdrawing film at a take up speed. The core and core contact layers are of a film forming polymer composition comprising at least 90 wt % of ethylene-based polymer. The ethylene-based polymer of the core layer has an average density of ethylene-based polymers equal to or higher than that of ethylene-based polymer of the core contact layers. The invention specifically concerns the feature that the core layer composition contains a portion of LDPE and the composition of the core contact layers contain a portion of a linear polyethylene so as to make the core layer more resistant to deformation than the skin layers.
U.S. Patent Application No. 2011/0003099 relates to the use of linear polyethylene having an MIR indicative of the presence of some long-chain branching having a density of 0.91 to 0.94 g/cm3 determined according to ASTM D4703/D1505, an I2.16 (MI) of from 0.05 to 1 g/10 min, and I21.6/I2.16 (MIR) of more than 35, the MI and MIR being determined according to ASTM 1238 D at 190° C., and a difference between the MD Tensile force based on ASTM D882-02 at 100% elongation and MD 10% and the offset yield of a reference film is defined herein having a thickness of 25 μm of at least 15 MPa. The invention also relates to coextruded film structures made using such linear polyethylene in the core layer of a multi-layer structure to provide easily processed, strong, highly transparent films.
As discussed above, there is a need therefore for a film and/or a method for making a film that can deliver low stiffness to a blown multilayer film without compromising optical properties, such as high clarity. One alternative to address the above stiffness problem is to apply a propylene-based elastomer to a conventional multilayer film. Applicant has discovered that during bubble blowing of a blown coextrusion process, introduction of a propylene-based elastomer into inner layers of the bubbles can create a strong adhesion between the two inner surfaces of the bubble when the bubble collapses. As a result, the two surfaces can turn into one layer serving as the core layer of the film. Thus, the number of layers in a film can be greater than the number of extruders in the coextrusion line, as represented by a function relation of (2n−1), with n being the number of extruders. Therefore, under a given film thickness and a limited scale of a coextrusion line with, for example, three extruders, a five-layer film comprising a propylene-based elastomer in the core layer can be produced to obtain lower stiffness, preferably combined with improved optical properties and other mechanical properties.